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Author Topic: Bird 4431 wattmeter question  (Read 3680 times)
VE7RWN
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Posts: 73




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« on: January 23, 2013, 10:23:18 AM »

I have a Bird model 4431 wattmeter, which is pretty much the same as a model 43, with the addition of an RF sampling port. For the first time, I tried using the port last night on my spectrum analyser while checking out my Alinco DX70 that I had just repaired. The spectrum analyser revealed an absolutely clean signal, with no harmonics. I was impressed! I thought the filters in this Alinco may have been better than those in my Yaesu rigs. I then went back to the normal setup I use when checking out a radio for harmonics, which is a high power 20 db attenuator, then into the spectrum analyser. With this setup, the normal harmonics appeared that I see on other rigs.
So the question is, is my model 4431 flaky, or am I missing something obvious?

Rob, ve7rwn
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AD4U
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« Reply #1 on: January 23, 2013, 12:55:48 PM »

I have no idea but will take a stab.  Do you know how much attenuation the Bird sampling port has?  If it is more than the 20dB of your external high power attenuator, then the additional attenuation MAY be attenuating the harmonics to the point they are not visible on the analyzer?

The only sampling port I have is a barrell connector with the innards knocked out.  I screw that onto a coax T and connect my old HP 141T to the other end of the gutted barrell connector.  The best I can measure (which is not very good) shows almost a constant 30dB attenuation all the way to 30 MHz.  Of course I do not use it to make any precision measurements.   Grin

Dick  AD4U
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WB2WIK
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« Reply #2 on: January 23, 2013, 03:34:00 PM »

I suspect a 20 dB attenuator isn't enough, and your spectrum analyzer is generating the harmonics you see.

You didn't say what analyzer you're using, but most of mine start to overload at anything >0 dBm (1 mW), although they'll "take" about +20 dBm without damage.  But "no damage" isn't the same as being usable, because at much over 0 dBm, the analyzer itself will create harmonics and junk that really isn't there.

If the rig runs 100W, -20dB from that is 1W, or +30 dBm.  That's an awful lot of power to pump into any spectrum analyzer I know of.

The Bird sampling port is normally adjustable in its decoupling, and I would guess you have it set to more like -40 dB or so, providing a more reasonable signal to the analyzer.
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KE3WD
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« Reply #3 on: January 23, 2013, 04:03:37 PM »

If that is a 70TH, indicating the FET outputs, they can be amazingly clean.  But they still have harmonics.  What doesn't? 

However, with the discrepancy you've found, there's got to be something else going on, check the specs for that Bird if you can, see if it is attenuated, or possibly something's not up to snuff in there, that sort of thing.


73
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W6OU
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« Reply #4 on: January 24, 2013, 10:31:28 AM »

Does your spectrum analyzer have an input attenuator?  If so, increase the input attenuation through several steps and note the shift in the spectrum display.  The fundamental and all of the harmonics should decrease by the same amount as attenuation is added.  If the harmonics decrease more than the fundamental then your analyzer is generating much of the harmonic spurs due to overload.
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VE7RWN
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« Reply #5 on: January 25, 2013, 12:30:05 PM »

Thanks to all who replied. The analyzer is an HP8285, which will take up to 2.5 watts input. Great piece of gear by the way, for the price on the used market. The Bird has an adjustable sampler from roughly 20db to 100db down, depending on frequency. When I look at the signal from the rig to the analyzer through a high power attenuator, say 20db and 500 watt capable, I see the normal harmonics, the first being approx 40 db down from the fundamental. If I use the sampling port on the wattmeter, which is effectively an adjustable Tee, I see the fundamental at roughly 60db above the noise floor, but no harmonics. The sensitivity of the analyzer can be set to whatever level is desired. I realise that the Bird could be knocking the edge off of the harmonics to the point that they are not visible, but with the fundamental as high as it is, I would have expected to see something.

Rob.
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G3RZP
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« Reply #6 on: January 25, 2013, 01:05:17 PM »

What's the bandwidth of the Bird slug, and what frequency are you operating at?
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VK2TIL
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Posts: 344




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« Reply #7 on: January 25, 2013, 03:10:56 PM »

An interesting problem.

From your description I think, but I'm not certain, that you are taking RF from after the 20dB attenuator for both tests and that you are using the same analyser settings for both the "attenuator" test and the "Bird" test; can you confirm that?

The Bird sampling port has a high-pass characteristic;

http://i46.tinypic.com/5z15k9.jpg

but this would tend to increase the measured harmonic level over its true value; you are seeing the opposite.

I don't think that the slug is relevant; I guess that the sampler pickup is in the line section of the 4431 and operates in the same manner irrespective of the type of slug that's inserted.

This problem interests me and I've been racking my meagre grey matter for an answer; I'm afraid that I don't have one.  Sad

 
« Last Edit: January 25, 2013, 03:13:19 PM by VK2TIL » Logged
WB2WIK
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« Reply #8 on: January 25, 2013, 03:32:49 PM »

Thanks to all who replied. The analyzer is an HP8285, which will take up to 2.5 watts input. Great piece of gear by the way, for the price on the used market. The Bird has an adjustable sampler from roughly 20db to 100db down, depending on frequency. When I look at the signal from the rig to the analyzer through a high power attenuator, say 20db and 500 watt capable, I see the normal harmonics, the first being approx 40 db down from the fundamental. If I use the sampling port on the wattmeter, which is effectively an adjustable Tee, I see the fundamental at roughly 60db above the noise floor, but no harmonics. The sensitivity of the analyzer can be set to whatever level is desired. I realise that the Bird could be knocking the edge off of the harmonics to the point that they are not visible, but with the fundamental as high as it is, I would have expected to see something.

Rob.

Did you try the "test" on the 8285 as recommended earlier?

That is, with the 20 dB attenuator and not the Bird sampler: View the harmonics on the analyzer.  Now, add another 10 dB of attenuation without changing anything else, and view the spectrum again.  Did both the intended signal and the harmonics all drop by exactly 10 dB?

If the harmonics dropped more than the primary signal, the analyzer was being overloaded.

Simple test.
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VE7RWN
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Posts: 73




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« Reply #9 on: January 26, 2013, 01:00:56 PM »

WB2WIK and VK2TIL: The tests have been done with EITHER the sampling port on the bird, OR the 20db high power attenuator in line, never with both. If I add another 10db high power attenuator in line to the 20 db, the results are as expected, a corresponding decrease in signal level of the fundamental and harmonics are displayed on the analyzer. Think of the Bird sampling port as a "Tee" in the line section, with an adjustable pickup in terms of proximity to the line section. Much like the old Philco-Sierra Wattmeters which had four power levels per slug.
G3RZP: The Bird slug is the 2-30mhz 250 watt slug. I am testing all the HF bands, 160mtrs through 10mtrs The Bird documents make no mention of the slug affecting the sampling port. I will try the test without a slug in the meter to see if it has any effect on the outcome.

Just to clarify the hookup:
Radio under test to high power attenuator to analyzer, or
Radio to Bird meter to dummy load, sampling port on Bird to analyzer.

Thanks, Rob..
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VK2TIL
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Posts: 344




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« Reply #10 on: January 26, 2013, 03:24:04 PM »

Thanks for the clarification.

I'm inclined to think that the analyser is overloading in the "attenuator" setup.

If you have 100w (+50dBm) TX output, the 20dB attenuator gives +30dBm at the analyser input.

This is very close to the +34dBm input maximum.

I'm not familiar with the 8285 and I had some difficulty in interpreting the data sheet but I imagine that the SA section is little different to any other SA; the difference would be that the 8285 has a lot more fixed attenuation at the mixer input.

For comparison, my SA has a specified maximum input of +20dBm but it begins to go non-linear at about 0dBm; I have tested this.  I have disposed of my other older HP SAs so I can't test them but memory says that they behaved similarly.

I have always tried to keep SA inputs around -20dBm or so, ie 20dB below distortion level and 40dB below maximum input power rating; applying this reasoning to your attenuator setup, there would be -6dBm into the SA requiring 56dB of attenuation.

As far as the "Bird" setup is concerned, the data sheet says that maximum power transfer/minimum attenuation (that's how I interpret the table I posted) is 57dB at 10 MHz.

That would result in a SA input power of -7dBm from 100w/50dBm; this is a good deal less than the +30dBm input from the "attenuator" setup and is close to ideal for a good measurement with the 8285 if my reasoning is correct.

The argument against my reasoning is as you said; the "attenuator" results seem believable whilst the "Bird" results seem too good to be true.

My work is mainly in micro-power; I've occasionally checked an amateur transmitter over the years but I don't remember one with (if I interpret your "Bird" results correctly) a second harmonic better than -60dBc.

I know nothing of the Alinco; perhaps it's capable of that as KE3WD remarked.

Try adding a lot more attenuation to make 50dB or so and see what happens to the second harmonic level.

You might like to try another method of "tapping" the TX output; a coupler;

http://i47.tinypic.com/315k37p.jpg

http://i45.tinypic.com/2vhw5ro.jpg

http://i50.tinypic.com/vdea7o.png

I built that one not long ago to use on the bench where BNCs are useful; I built this one;

http://i47.tinypic.com/3480g06.jpg

years ago for use with UHF connectors.



 
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WB2WIK
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« Reply #11 on: January 28, 2013, 10:11:21 AM »

WB2WIK and VK2TIL: The tests have been done with EITHER the sampling port on the bird, OR the 20db high power attenuator in line, never with both.

I fully understood that from the beginning.

Quote
If I add another 10db high power attenuator in line to the 20 db, the results are as expected, a corresponding decrease in signal level of the fundamental and harmonics are displayed on the analyzer.

That's new information.  Frankly, I'm surprised to hear this.

Quote
Think of the Bird sampling port as a "Tee" in the line section, with an adjustable pickup in terms of proximity to the line section.

I have used the Bird sampling port meters and never found they had a low-pass nor a bandpass characteristic, so the whole story is surprising!  Based on what you've reported, it sounds like yours is demonstrating a low-pass characteristic for some reason.


 
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VK2TIL
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Posts: 344




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« Reply #12 on: February 02, 2013, 08:44:40 PM »

Looks as if the OP has moved-on; pity, I would like to know the answer.

I note that the Alinco spec is for harmonics better than -50dBc (HF except for 10m) and -60dBc (6m);

http://i47.tinypic.com/ezo6c.jpg

It's entirely possible that the "Bird" test is the one that gives the correct result, esp. if the reported results were measured at 50 MHz.
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VE7RWN
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Posts: 73




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« Reply #13 on: February 03, 2013, 07:20:38 PM »

A chaotic work schedule has kept me from checking back in.
I purchased the Bird used, therefore it may have some issues.
I spoke with a friend of mine who is a hardware engineer, and he suggested I sweep the Bird with the HP, since it has a built in tracking generator. It will be a bit before I am back at the test bench, but I will post the results once I have more info.

Tnx, rob..
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VK2TIL
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Posts: 344




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« Reply #14 on: February 03, 2013, 07:32:19 PM »

Thanks for that; I would like to know the outcome.
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